President Donald Trump said he is seriously considering naming US State Department spokeswoman Heather Nauert as the next US ambassador to the United Nations.

On Thursday, Trump said Nauert is at the top of his list and a decision would likely be made next week.

"She is under very serious consideration," Trump told reporters at the White House after announcing planned changes to US immigration policy.

"She's excellent. She's been with us for a very long time. She's been a supporter for a long time ... We'll probably make a decision next week."

Fox News, citing a top Trump administration official, reported earlier on Thursday that Nauert had already been offered the job. ABC News also reported that it was unclear whether Nauert had accepted the president's nomination.

Trump would only say Nauert was in the running for the UN post, however, and he didn't reveal which other candidates he was considering.

"We have a lot of people that want the job," he told reporters.

If appointed, Nauert would replace Nikki Haley, who is expected to end her tenure as Washington's envoy to the UN by the end of the year.

Haley, a staunch defender of Israel at the UN, stepped down last month, citing a desire to get new people into politics. "I think you have to be selfless enough to know when to step aside and let someone else do the job," she said at the time.

No diplomatic experience

Nauert has been the State Department spokeswoman since April 2017, starting her role under then-Secretary of State Rex Tillerson and currently serving under Mike Pompeo. She was also named acting undersecretary for public diplomacy and public affairs earlier this year.

She previously worked as a correspondent and anchor for Fox News, appearing on the programme Fox & Friends, a favourite of the president.